Some Consistency Conditions for Dynamic Traffic Assignment Problems

This paper explores some critical issues in modeling network traffic flow for predictive user optimal dynamic traffic assignment. They include the role of FIFO (first-in-first-out) in link, path and network traffic dynamics, minimal set representation of link traffic evolution, and consistency conditions for link models. It is found that (1) link FIFO plays a central role in modeling link, path and network traffic flow evolution, and (2) the dimension of a minimal state set that adequately describes link traffic flow under FIFO is two. Moreover, two sufficient FIFO conditions are provided, and pitfalls in using these conditions to enforce FIFO are pointed out. Finally, ramifications of these findings to DTA modeling are also discussed.     

A Comparative Study of Some Macroscopic Link Models Used in Dynamic Traffic Assignment

As network loading forms the basis of many dynamic traffic assignment solution algorithms, we consider in this paper four macroscopic link models that are widely used in DTA studies as the building blocks of certain network loading procedures. For the same generic link, the four models are solved and evaluated numerically for various link inflow profiles under a single-destination framework. And the characteristics of each model is analyzed and compared with others from multiple perspectives such as algorithmic implementation, model calibration, model accuracy, computational time and memory consumption. It is our purpose to find the best link model for the development of realistic and efficient network loading procedures. Moreover, since model discretization and calibration substantially influence a link models performance, both issues are carefully addressed in the paper.     

Analytical Models of the Dynamic Traffic Assignment Problem

A review of analytical formulations of the dynamic traffic assignment problem is presented, focusing on the authors' experience with variational inequality approaches. Solution algorithms and computational issues requiring additional study are discussed.     

A Decomposition Scheme for System Optimal Dynamic Traffic Assignment Models

In this paper, a Dantzig-Wolfe decomposition based solution algorithm is developed for the linear programming formulation introduced by Ziliaskopoulos (2000) for System Optimal Dynamic Traffic Assignment problem. The algorithm takes advantage of the network structure in the constraint set of the formulation: the sub-problem is formulated as a minimum-cost-flow problem and the master as a simpler linear programming problem, which allows DTA to be solved more efficiently on meaningful networks. The algorithm is tested on an example network and its performance is analyzed.     

Continuous and Discrete Trajectory Models for Dynamic Traffic Assignment

A continuous trajectory model is presented in which transportation networks are represented as topological constructs. The general formulation enhances existing analytic dynamic traffic assignment models by incorporating continuous single-link traffic flow models in a general, coherent, and relatively intuitive manner. Specific exact formulation based on a simplified kinematic wave traffic flow model with physical queues is presented as well.A discrete trajectory model is proposed as an approximation of the continuous model. The discrete model provides wide flexibility in choosing the level of aggregation with respect to time intervals, ranging from several hours, as typical in current practice of long-term travel forecasting models, to one second or less, as in microscopic simulations. An algorithm to find discrete approximate solutions is presented as well as accuracy measures to evaluate them. The effect of time resolution on model performance is examined by a numerical example.     

Foundations of Dynamic Traffic Assignment: The Past,the Present and the Future

Dynamic Traffic Assignment (DTA) has evolved substantially since the pioneering work of Merchant and Nemhauser. Numerous formulations and solutions approaches have been introduced ranging from mathematical programming, to variational inequality, optimal control, and simulation-based. The aim of this special issue is to document the main existing DTA approaches for future reference. This opening paper will summarize the current understanding of DTA, review the existing literature, make the connection to the approaches presented in this special issue, and attempt to hypothesize about the future.     

面向动态交通分配的路段费用函数建模研究

在交通分配的研究中,车辆在路段上的行驶费用是决定网络交通流分布的关键因素之一。论文面对动态交通分配建模需要,在网络饱和条件下,对车辆在路段上的行驶时间进行了分析和建模。详细讨论了饱和交通网络中考虑容量约束下的车辆连续行驶时间和路口延误时间,在排队延迟模型中考虑了车辆的实际长度及由此而带来的影响,建立了离散的函数模型。最后给出计算实例,对比了点排队和实际排队的不同。     

The CONTRAM Dynamic Traffic Assignment Model

CONTRAM is a computer model of time-varying traffic in road networks, which takes as input the network definition and time-varying demand for travel between a set of origin and destination zones, and outputs the resulting network flows, routes and travel times. It combines a macroscopic time-sliced traffic model with disaggregate dynamic assignment of traffic, so is intermediate between macroscopic equilibrium and microscopic models. The paper details the methods used, including time-dependent queuing which plays a central role, and the treatment of network definition, user classes, road capacities, signals and coordination, vehicle emissions, Intelligent Transport Systems and research lines.     

改进粒子群算法在动态交通分配问题中的应用

在分析常规粒子群优化算法的基础上,针对动态交通分配问题,采用一种带变异因子的改进的粒子群优化算法,并根据粒子群的群体适应度标准差和理论最优值,给出收敛判断的依据。实验表明该算法具有简单、高效、快速的特点。     

Research on Combined Dynamic Traffic Assignment and Signal Control

This paper presents a generalized bi-level programming model of combined dynamic traffic assignment and traffic signal control,and especially analyzes a procedure for determining the equilibrium queuing delays on saturated links for dynamic network signal control satisfying the FIFO (first-in-first-out)rule.The chaotic optimal algorithm proposed in this paper can not only present the optimal signal settings,but also calculate,at each interval,the link inflow rates and outflow rates for the dynamic user optimal problem,and provide real-time information for the travelers.Finally,a numerical example is given to illustrate the application of the proposed model and solution algorithm, and comparison shows that this model has better system performance.     

最优动态配流问题的离散模型及其解

对一般路网建立了具有良好结构形式的离散动态配流模型，该模型在多起点单讫点路网中的最优解满足交通流守恒约束和动态平衡原则。在提出了完全不同路段概念的基础上，对最优解的奇异性进行了分析，得到稳态最优解为非奇异解的结论。给出的算例验证了结论的正确性。     

基于改进遗传算法的动态交通分配优化研究

传统的优化算法大多数由于计算量大或者容易使性能指标落入局部最优值而严重制约了模型的应用与发展,而用遗传算法等新的智能算法求解则会很简洁和方便。文章针对遗传算法在应用中存在的局限性,采用了小生境技术的遗传算法,结合精英保留策略、种群多样性保持方案、新的适应度值标定方式等改进遗传算法。通过对动态交通分配的特点分析,建立了动态交通分配模型,利用改进的遗传算法对模型进行求解。仿真结果体现了动态交通分配模型的有效性和改进遗传算法的优越性,大大提高了动态交通分配模型的实用价值。     

A Hybrid Deployable Dynamic Traffic Assignment Framework for Robust Online Route Guidance

Randomness in time-dependent origin-destination (O-D) demands and/or network supply conditions, and the computational tractability of potential solution methodologies are two major concerns for the online deployment of dynamic traffic assignment (DTA) under real-time traffic management systems. Most existing DTA models ignore these concerns and/or make unrealistic assumptions, precluding their online applicability. In this paper, a hybrid approach consisting of offline and online strategies is proposed to address the online stochastic dynamic traffic assignment problem. The basic idea is to address the computationally intensive components offline, while efficiently and effectively reacting to the unfolding conditions online. The offline component seeks a robust initial solution vis-à-vis randomness in O-D demands using historical O-D demand data. Termed the offline a priori solution, it is updated dynamically online based on unfolding O-D demands and incidents. The framework circumvents the need for accurate O-D demand and incident likelihood prediction models online, while exploiting historical O-D demand and incident data offline. Results of simulation experiments highlight the robustness of the hybrid approach with respect to online variations in O-D demand, its ability to address incident situations effectively, and its online efficiency.     

Efficient Discretisation for Link Travel Time Models

In the literature, two different models have been used to compute link travel times in dynamic traffic assignment (DTA), and elsewhere we investigated how these are affected by discretising the link length. Here we consider discretising time as well as space (the link length). We vary the discretising of time with spatial discretisation held fixed, and vice versa, and also vary both together. The results show that coordinated discretisation is usually the most efficient in approximating the limit solution (continuous time, continuous space) and, even when it is not the most efficient, it has other advantages. The results have implications for algorithms for DTA and for the choice of discrete versus continuous time models. For example, refining the discretisation of time (without refining it for space) can make the solution less accurate, so that in the widely used whole-link models (i.e. without spatial discretisation) it is more efficient to use the largest feasible time steps, close to the link travel time.     

GIS and ITS Traffic Assignment: Issues in Dynamic User-Optimal Assignments

Dynamic traffic assignment (DTA) is at the heart of much ITS research. Assigning traffic, whether for planning purposes or for real time route guidance, is a difficult problem. Recent advances in user-optimal dynamic traffic assignment have built on the methods developed for static user-optimal assignments. Since assignment models are complex, they will not use many of the network analysis functions found in commercial GIS packages. Custom software will have to be developed. In this paper we explore the problems faced in solving static and dynamic assignments and relate those problems to information that is likely to be based in a traffic control centers GIS database. Because of the size of the problem and the need for faster-than-real-time analysis, how and when data is transferred between a GIS to analysis modules is important. Further, many approaches for GIS software design and spatial data handling, such as OOP and dynamic segmentation, may impose too much overhead to be of much use in time-sensitive ITS applications.     

Risk-Averse Traffic Assignment with Elastic Demands: NCP Formulation and Solution Method for Assessing Performance Reliability

How reliable the transport network is greatly affects the mobility and economy of a country. To assess the reliability of the network accurately and design a reliable transport network properly, one should consider the risk taking and elastic behaviour of travel demand. This paper develops a Nonlinear Complementarity Problem (NCP) formulation considering this behaviour. The NCP formulation is transformed to a smooth and unconstrained mathematical program via a Fischer function and then solved by the proposed modified αBB (branch and bound) algorithm, while guaranteeing finite convergence to a point arbitrarily close to the global minimum (ɛ-convergence). To speed up the computation, a sampling method is suggested. A small numerical study is set up to illustrate the properties and the performance of the algorithm. The results show that the bound improving strategy, the sampling size on estimating the value of α for the convex lower bounding function, and choice of α have a strong influence of the rate of convergence.     

WDM光传送网中基于链路状态等级的动态路由与波长分配算法

提出了两种基于链路状态分级的路由与波长分配算法．分别为链路等级优先的动态路由与波长分配算法（LFLS-DRAW）和满足等级要求的动态路由与波长分配算法（RLSDRAW）算法．理论分析表明，基于链路状态分级的路由与波长分配算法的计算复杂度与同类算法相当，且与其它算法相比具有一定的优越性．仿真结果表明．基于链路状态分级的路由与波长分．配算法在计算时间和平均路由跳数相近的情况下．所选择路径的路由状态等级较高．具有较强的负载均衡能力，可以有效降低网络的拥塞概率．     

基于仿真和蚁群算法的公交动态交通规划研究

针对越来越复杂的道路交通系统,研究其中的动态交通规划问题。以达到对交通进行合理规划的目的。采用计算机仿真技术构建动态交通规划模型,应用蚁群算法解决基于仿真的动态交通规划优化问题。在所建模型的基础上,通过蚁群算法进行求解。实验结果令人满意。仿真方法可以将普通动态交通规划模型无法反映的随机因素考虑在内,使得动态交通规划的结果更加具有现实中的指导意义。将优化技术嵌入到仿真过程中。在仿真环境下使输出响应不断地得到改进,从而实现道路交通系统性能的优化。数据实例表明,该方法是正确的、可行的、有效的,可以为实际的道路交通规划提供有力地决策支持。